The present invention relates to a gallium oxide (Ga2O3)/zinc oxide (ZnO) sputtering target (GZO target) for obtaining a transparent conductive film capable of maintaining a favorable optical transmission factor and conductivity. Further, it relates to a method of forming a transparent conductive film with the use of the target and relates to a transparent conductive film formed thereby.
Conventionally, as a transparent conductive film, an ITO film (i.e., tin-doped indium oxide film) is superior in transparency and conductivity, and is widely used as a transparent electrode (film) of display devices such as a liquid crystal display and an electro luminescence display, as well as in solar batteries. Nevertheless, since indium, which is the primary component of ITO, is expensive, there is a problem in that the manufacturing cost will increase.
Due to the foregoing reasons, the usage of a GZO film as a substitute for the ITO film is being proposed. This GZO film has the advantage of being inexpensive, since it is a zinc oxide film having gallium oxide (Ga2O3)/zinc oxide (ZnO) as its primary component. The GZO film is known to encounter a phenomenon of increased conductivity due to the oxygen defect of ZnO, which is the primary component thereof, and the increased use of such GZO film can be realized if the film characteristics such as conductivity and optical transparency can approximate the film characteristics of an ITO film.
As a method of forming the GZO film, the sputtering method is primarily used, and, in particular, direct current (DC) sputtering, radio frequency (RF) sputtering or magnetron sputtering is used from the perspective of operability and film stability.
Formation of a film based on the sputtering method is conducted by physically colliding positive ions such as Ar ions to a target disposed on a negative electrode, using such collision energy to discharge a material for configuring a target, and laminating a film having roughly the same composition as the target material on a substrate on the positive electrode opposite to the target.
The coating based on the foregoing sputtering method is characterized in being able to form a thin film of angstrom units to a thick film of several ten μm with a stable deposition speed by adjusting the processing time and power supply.
Several proposals have been made regarding a sintered sputtering target for forming this kind of GZO film, and a transparent conductive film formed thereby.
For example, Patent Document 1 proposes a zinc oxide sintered target, which is free from abnormal electrical discharge and capable of forming a stable thin film, as a part of the invention. In Patent Document 1, a Ga2O3/ZnO target sintered compact is used as a part of the target material, and zinc oxide in which titanium oxide, germanium oxide, aluminum oxide, magnesium oxide, indium oxide, and tin oxide are selectively added at 1 to 5 weight % is used as the primary component.
Patent Document 2 proposes a GZO sintered sputtering target, which is free from abnormal electrical discharge and capable of forming a stable thin film. In Patent Document 2, technology is proposed for improving the density by pulverizing the powders of zinc oxide and gallium oxide to a grain size of 1 μm or less, adjusting the sintering temperature at 1300 to 1550° C., and performing sintering while introducing oxygen.
Patent Document 3 proposes a GZO sintered sputtering target with a high transmission factor and a low resistance value and in which the occurrence of an abnormal electrical discharge is rare over a long period of time. In Patent Document 3, proposed is a ZnO sintered compact in which Ga is added at 3 to 7 atomic %, and a third element selected from Al, B, In, Ge, Si, Sn, and Ti is added at 0.3 to 3 atomic %.
Patent Document 4 proposes technology of performing sputtering in an atmosphere of hydrogen gas and inert gas in order to prevent changes in the electrical characteristics and optical characteristics due to zinc oxide reacting with moisture.
Generally speaking, a major problem in forming a GZO film is that minute protrusions called nodules occur in the erosion portion of the target surface due to sputtering, coarse grains (particles) float in the sputtering chamber caused by the abnormal electrical discharge and splash due to such nodules, and such particles adhere to the formed film and cause deterioration in quality. Further, the foregoing abnormal electrical discharge will cause an unstable plasma discharge, and there is a problem in that stable deposition cannot be realized.
Therefore, upon forming a conductive film on a substrate, it is necessary to periodically remove the nodules occurring on the sputtering target, and there is a problem in that this significantly deteriorates the productivity. Thus, a target which does not generate nodules and an abnormal electrical discharge phenomenon is in demand.
In particular, displays recently tend to be enlarged, and large-area deposition is required. Thus, a target capable of stable deposition is in particular being demanded.
In the foregoing Patent Documents, the problem of abnormal electrical discharge is being pointed out. Patent Document 1 described above suggests that titanium oxide, germanium oxide, aluminum oxide, magnesium oxide, indium oxide, and tin oxide are selectively added at 1 to 5 weight % as measures for reducing such abnormal electrical discharge. Patent Document 3 suggests that a third element selected from Al, B, in, Ge, Si, Sn, and Ti is added at 0.3 to 3 atomic %.
Each invention of the foregoing documents tries to prevent the abnormal electrical discharge by increasing the density of the sintered compact and reducing the holes in the sintered compact. Nevertheless, even with the use of these additive materials, there is a problem in that the sintered density cannot be sufficiently increased, and the bulk (volume) resistance value remains high.
Further, although it is possible to improve the manufacture process of the target, a complex manufacture process will result in increased costs. In addition, when attempting to increase the density by improving the sintering method or device, there is a problem in that the equipment must be enlarged, and this cannot be considered to be an industrially efficient method.
In addition, a GZO sintered target has drawbacks in that the bulk resistance value and density will change significantly and lack stability depending on the sintering conditions, and the composition will change since it easily reacts with the setter or the like during sintering.
Comprehensively, by adding trace elements, or by changing the component composition of the GZO sintered compact, it is possible to improve the target density, prevent the formation of nodules, and inhibit the abnormal electrical discharge phenomenon and generation of particles, and this would be a simple and effective method. Nevertheless, since change in the component composition will aggravate the bulk resistance value of the target and will not necessarily contribute to an improvement in the sintered density, there is a problem in that the examples described in the foregoing Patent Documents are insufficient as measures in overcoming the conventional problems.
[Patent Document 1] Japanese Patent Laid-Open Publication No. H10-306367
[Patent Document 2] Japanese Patent Laid-Open Publication No. H10-297964
[Patent Document 3] Japanese Patent Laid-Open Publication No. H11-256320
[Patent Document 4] Japanese Patent Laid-Open Publication No. 2002-363732